About 10% difference is typical. There is lots of lamina data available in CMH-17 Volume 2 (available at sae.org).

Compression modulus is not difficult to measure, though generally requires use of strain gages due to short gage lengths required to prevent specimen buckling.

Are the facesheets of your sandwich panels precured or cocured? do the panels use honeycomb core? Both tension and compression moduli will be reduced for sandwich panel facesheets that are cocured over honeycomb core where the facesheet dimples into the cells.

Awesome - that's super helpful. It sounds like there is a good chance some of our coupons are buckling if we are measuring 20-30% knockdowns. I'll circle back with our M&P team to try to confirm that.

The sandwich panel facesheets would be cocured to either honeycomb or foam.

For carbon fiber composites, modulus should be calculated as secant value over 0.001 to 0.004 in/in strain range, so should be well below onset of buckling (unless the specimen/fixture is complete junk).

Ask them for the load-strain plots for both tension and compression tests.

This reminds me of the time I was asked to get involved in helping solve a mystery as to why a new material was not performing well in compression. It turned out that the compression test fixture used linear ball bushings on 0.5 inch rods to keep the two halves in alignment during the test. No one noticed that the bushings had been pushed up into their mounting holes so that there was little engagement between the bushings and guide rods. Thus the two halves of the test fixture were not staying perfectly aligned during testing, leading to low compression numbers.